The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Known spark-ignition (SI) engines introduce an air/fuel mixture into each cylinder which is compressed in a compression stroke and ignited by a spark plug. Known compression ignition engines inject pressurized fuel into a combustion cylinder near top dead center (TDC) of the compression stroke which ignites upon injection. Combustion for both SI and compression ignition engines involves premixed or diffusion flames controlled by fluid mechanics.
SI engines can operate in combustion modes, including a homogeneous SI combustion mode and a stratified-charge SI combustion mode. SI engines can be configured to operate in a homogeneous-charge compression-ignition (HCCI) combustion mode, also referred to interchangeably as controlled auto-ignition combustion, under predetermined speed/load operating conditions. The controlled auto-ignition (HCCI) combustion includes a distributed, flameless, auto-ignition combustion process that is controlled by oxidation chemistry. An engine operating in the controlled auto-ignition (HCCI) combustion mode has a cylinder charge that is preferably homogeneous in composition, temperature, and residual exhaust gases at intake valve closing time. Controlled auto-ignition (HCCI) combustion is a distributed kinetically-controlled combustion process with the engine operating at a dilute air/fuel mixture, i.e., lean of an air/fuel stoichiometric point, with relatively low peak combustion temperatures, resulting in low nitrous oxide (NOx) emissions. The homogeneous air/fuel mixture minimizes occurrences of rich zones that form smoke and particulate emissions.
Controlled auto-ignition (HCCI) combustion depends on factors such as cylinder charge composition, temperature, and pressure at intake valve closing. Hence, the control inputs to the engine must be carefully coordinated to ensure auto-ignition combustion. Controlled auto-ignition (HCCI) combustion strategies may include using an exhaust recompression valve strategy. The exhaust recompression valve strategy includes controlling a cylinder charge temperature by trapping hot residual gas from a previous engine cycle by adjusting valve close timing. In the exhaust recompression strategy, the exhaust valve closes before TDC and the intake valve opens after TDC creating a negative valve overlap (NVO) period in which both the exhaust and intake valves are closed, thereby trapping the exhaust gas. The opening timings of the intake and exhaust valves are preferably symmetrical relative to TDC-intake. Both a cylinder charge composition and temperature are strongly affected by the exhaust valve closing timing. In particular, more hot residual gas from a previous engine cycle can be retained with earlier closing of the exhaust valve leaving less room for incoming fresh air mass, thereby increasing cylinder charge temperature and decreasing cylinder oxygen concentration. In the exhaust recompression strategy, the exhaust valve closing timing and the intake valve opening timing are measured by the NVO period.
Engine airflow is controlled by selectively adjusting position of the throttle valve and adjusting opening and closing of intake valves and exhaust valves. On engine systems so equipped, opening and closing timings of the intake valves and exhaust valves are accomplished using a variable valve actuation system that includes variable cam phasing and a selectable multi-step valve lift, e.g., multiple-step cam lobes which provide two or more valve lift positions. In contrast to the throttle position change, the change in valve lift position of the multi-step valve lift mechanism is a discrete change, and not continuous.
When an engine operates in a controlled auto-ignition (HCCI) combustion mode, the engine control includes lean or stoichiometric air/fuel ratio operation with the throttle wide open to minimize engine pumping losses. When the engine operates in the SI combustion mode, the engine control includes stoichiometric air/fuel ratio operation, with the throttle valve controlled over a range of positions from 0% to 100% of the wide-open position to control intake airflow to achieve a stoichiometric air/fuel ratio.
One known fuel control strategy includes cutting off fuel to all engine cylinders when an operator releases an accelerator pedal during ongoing vehicle operation, referred to as a fuel cutoff (FCO) event. When the operator subsequently tips into the accelerator pedal, the engine is re-fueled and re-fired. Re-firing the engine when operating in the controlled auto-ignition (HCCI) mode uses residual heat from a previous combustion cycle to initiate auto-ignition combustion. Re-firing the engine subsequent to a fuel cutoff event when operating in the controlled auto-ignition (HCCI) mode can result in unstable combustion, as residual heat from a previous combustion cycle is not available.
One method to initiate re-firing in the controlled auto-ignition (HCCI) mode at arbitrary engine loads includes injecting a small portion of the total engine fuel mass close to the spark plug just prior to spark ignition timing to initiate flame propagation in an overall lean combustion charge mixture. This method of re-firing may result in excess NOx emissions for a few engine cycles due to a high amount of flame burning when large amount of fuel is injected.